The ceramic coating of metals, such as porcelain enameling, is well known in the art. A preferred process for depositing a ceramic coat is by electrophoresis as described, for example, in U.S. Pat. No. 3,575,838 to Hughes. Ceramic coating by electrophoresis is generally regarded as superior to conventional dipping or spraying techniques, because electrophoresis is more rapid and more effectively coats all surfaces of a workpiece, including its edges.
However, not all metals can be coated by electrophoresis because not all metals are sufficiently electrochemically active. By the term "electrochemically active" is meant the ability to pass metal ions into an electro-conducting solution during current flow. Examples of metals which are sufficiently electrochemically active when immersed in an electrophoretic medium and, therefore, can be coated by electrophoresis are mild steel and sandblasted cast iron. On the other hand, notable among metals which are not sufficiently electrochemically active and which, therefore, cannot be readily coated by electrophoresis are alloys containing nickel or chromium or alloys of nickel or chromium in appreciable amounts, for example, at least 10% by weight of such a metal or alloys of such metals. These two elements appear to be largely responsible for the failure of a metal containing either one of them to be suitably coated by electrophoresis. The various stainless steels are examples of such alloys as well as the so-called super alloys, such as those sold under the trademarks, Inconel, Monel, Incoloy, and Hastelloy.
It is possible to deposit ceramic coats on alloys like stainless steels other than by electrophoresis. For example, stainless steel parts have been treated with an aqueous slurry of ceramic particles and then fired. Such stainless steel parts include those of intricate electric circuit systems, jet engine parts, furnace tooling, automotive exhaust trains, and the like. Ceramic coats are usually applied to stainless steel and other super alloys to protect them from corrosion and oxidation at elevated temperatures and/or corrosive atmospheres.
However, prior techniques for ceramic coating of stainless steel and other like alloys other than by electrophoresis are also fraught with problems. For instance, the various methods of preparing stainless steels for ceramic coating such as etching, sandblasting, etc., have not been entirely satisfactory. Although sandblasting is the best method at present and is the only known technique to be commercially successful in making a stainless steel surface suitable for ceramic coating, many times even this process falls far short of being completely successful. Sandblasting is expensive and also a limited operation, because many formed parts have areas that are not accessible to the flow of abrasive material. Even when a part is successfully sandblasted, the quality of a subsequently applied ceramic coating can not be guaranteed. Because of the problems met, especially spalling, ceramic coating of stainless steels has often been considered a last resort.
In addition, even if stainless steel parts become ceramically coated, they previously have not been free of problems later encountered in use, especially when such parts are subjected to elevated temperatures. Ceramic coatings by nature of their composition inherently have a low thermal expansion. Stainless steel alloys in general have a thermal expansion much higher than mild steels. As a result, there is a far greater expansion differential between a stainless steel-ceramic coat system than a mild steelceramic coat system. This difference in thermal expansion limits the actual thickness of most ceramic coats to approximately one mil. Heavier or thicker applications only result in spalling, while a thinner application of a ceramic coat does not provide the desired protection. When the thickness of the ceramic coat becomes critical, application of a ceramic slip as by dipping is usually not feasible and spray applications are necessary. Sharp edges and small radii are also critical application points, since these areas amplify the expansion problems due to differences in thermal expansion rates.